The present invention relates to an automatic frequency control apparatus, and in particular to an automatic control apparatus, provided for a demodulator in a radio communication apparatus, that can employ the carrier phase error of a received signal, obtained through digital phase modulation or digital frequency modulation, to quickly and stably converge, at a desired frequency, the frequency of a carrier that is locally oscillated.
Various techniques for reducing power consumption are employed for demodulation of a radio communication apparatus used for a radio communication system. One of the techniques involves the use of an automatic frequency control (AFC: Automatic Frequency Control) apparatus that detects the phase of a received signal obtained through digital phase modulation or digital frequency modulation, and that employs the carrier phase error of the received signal to converge, at a desired frequency, the frequency of a carrier that is processed by a local oscillator.
In a conventional automatic frequency control apparatus, along a time axis a gain unit determines a loop gain to be multiplied by phase error data, and when the loop gain is high, a frequency deviation to be corrected is increased and the frequency quickly follows up a desired frequency, while the operating stabilization is low. When the loop gain is low, the frequency deviation to be corrected is reduced and the frequency slowly follows up a desired frequency, while stabilization of the operation is increased.
Since regardless of the magnitude of the phase error, the conventional automatic frequency control apparatus varies, along the time axis, the loop gain to be multiplied by the phase error data, it is difficult to obtain a correction value that is consonant with an actual change in the phase error.
That is, in accordance with a time-transient instruction from a timer or a controller such as a DSP or a CPU, at the initial synchronization time, whereat it is estimated that the phase error is the greatest, the gain unit either increases the loop gain that is to be multiplied, or reduces the loop gain after a predetermined period of time has elapsed and a range is reached wherein the frequency is stabilized. Therefore, since the high loop gain is multiplied even through the phase error at the initial synchronization time is small, an extended period of time is required before the stable frequency range is reached. Further, since in the stable range, following the elapse of the predetermined time, a low loop gain is multiplied, even when the phase error is increased due to an anticipated change in the transmission condition, an extended period of time is required before the frequency can again be converged at the desired frequency.
Further, since the conventional automatic frequency control apparatus performs the loop process to correct the frequency of the carrier for which local oscillation is provided, phase error vibration may occur due to the loop delay.
That is, for a signal for which the phase is adjusted by a mixer, a loop delay occurs that continues until the correction of the phase error is again reflected to the mixer, and depending on the value of the initial phase error, convergence of the frequency occurs while the phase error is vibrated while being repetitively increased and decreased, when the loop gain to be multiplied is equal to or greater than a specific value. Because of the vibration, the convergence time is extended considerably, and even when the loop gain is increased until it is equal to or greater than a specific value, it is not possible to reduce the convergence time.